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Presence of potentially toxic microalgae confirmed in La Concha Bay

The proliferation of the Ostreopsis ovata algae is no cause for alarm, but it is advisable to continue taking measurements, according to Laurenns Balparda, a researcher at the University of the Basque Country (UPV/EHU)

  • Research

First publication date: 11/03/2025

Yago Laurenns-Balparda collecting samples. | Photo: Aner Etxebarria

The Fluvial Ecology research group has analysed the dynamics of the Ostreopsis microalgae in La Concha Bay. They conclude that these micro-organisms grow more abundantly in Ondarreta than in La Concha because the rocky seabed with macroalgae encourages their growth. The researchers point out that, although some species of these microalgae may be toxic to humans, their large presence does not mean that bathing is dangerous.

Ostreopsis is a microscopic alga that lives in shallow waters. One of its species is toxic and can cause symptoms such as coughing, fever, skin irritations or even mild respiratory problems. Although they originate in tropical regions, the warming of the oceans has turned areas such as the Basque coast into suitable habitats for them to grow. In the Bay of Biscay they were first detected in 2007. Back then, their presence was sporadic, but nowadays it is common for these microalgae to bloom massively along our coastline during the summer. They do not have to cause problems, but in 2020 the first cases of poisoning were recorded in Lapurdi-Labourd, and in 2021, in Donostia/San Sebastian.

In this context, a team from the UPV/EHU’s phytoplankton laboratory studied the proliferation of Ostreopsis in La Concha Bay: “We analysed their presence in two locations: in the west of Ondarreta and in the east of La Concha. The aim was to find out about their development in both locations and to detect the factors that led to the huge growth of the microalgae in Donostia,” explained the researcher Yago Laurenns-Balparda.

The study revealed that Ostreopsis is more abundant in Ondarreta and its authors concluded that this is due to the type of substrate: “The two locations where we took samples are quite similar in terms of temperature, current, wind and salinity. What distinguishes them is the substrate: the seabed of Ondarreta is completely covered by rocks of varying sizes where there is a large amount of macroalgae, which is one of the favourite places for Ostreopsis to grow. By contrast, almost the entire seabed of La Concha is sand; there are few rocks or macroalgae. So, the substrate does not encourage the development of Ostreopsis.”

Laurenns-Balparda pointed out that the fact that this microalga is abundant on our coasts does not mean that bathing from these beaches is always dangerous: “In fact, even though the samples collected in the summers of 2022 and 2023 indicated a massive presence of Ostreopsis in La Concha Bay, no cases of poisoning were recorded on the beaches of Donostia during those years. It is advisable to continue taking measurements to find out the level of concentration of these algae and to keep the situation under control, but their abundance is not always a cause for alarm. Just because there is a lot of Ostreopsis doesn't mean that toxicity is high.”

Presence of potentially toxic species ovata confirmed for the first time in La Concha Bay

Besides corroborating the importance of the type of substrate as a determining factor in encouraging the massive growth of Ostreopsis, the research carried out by the UPV/EHU has served to confirm that, of the thirteen species that exist, the toxic ovata is present in La Concha Bay. This is something new, given that until now only research in which the harmless species Siamensis was detected have been published. Laurenns-Balparda pointed out that “in the past it could be assumed or taken for granted that ovata was also present, because it is a priori the only toxic species of Ostreopsis. However, it could not be confirmed. Our study was the first to prove it”.  

Even so, there are still questions to be answered, since although the study showed that Siamensis and Ovata coexist in La Concha and Ondarreta, it was not able to determine the proportion in which each of the species is present: “We weren’t able to do this because they are very similar to each other and cannot be differentiated even under a microscope. However, thanks to molecular studies, we did manage to confirm that many of the strains of cells that we isolated from the samples collected on the beaches were ovata, but these analyses do not allow us to know which of the two species is more abundant,” explained Laurenns-Balparda.

The UPV/EHU biologist emphasises the importance of conducting further research to gain a better understanding of the dynamics of these microalgae, “to get more comprehensive knowledge about what other factors may affect their growth and toxicity, and to be able to implement some technology that will help to determine which species predominates”.

Additional information

Yago Laurenns-Balparda is a researcher in the Department of Plant Biology and Ecology in the Faculty of Science and Technology - Bizkaia. He belongs to the Fluvial Ecology research group and his lines of study focus on phytoplankton. The work on Ostreopsis in Donostia is part of a broader project that seeks to analyse the presence of these microalgae in more locations along the Basque coastline.

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